The present invention relates to endoscopic examining apparatus, to a probe useful in such apparatus, and to a method of making an electrical coil assembly useful in such a probe. The invention is particularly useful in an endoscope in MRI (magnetic resonance imaging) apparatus, and is therefore described below with respect to this application, although it will be appreciated that various aspects of the invention could advantageously be used in other applications as well.
Gastrointestinal Endoscopy:
Endoscopy is a common minimally invasive diagnostic and therapeutic procedure. Insertion of endoscopes into a body cavity or lumen (e.g., the gastro-intestinal tract, the genito-urinary system, the brain ventricles), or produced cavity or lumen (e.g. by laparoscopy) enables access to internal organs for visual inspection, for tissue sampling (biopsy), and for management of pathologies. A limiting factor in the use of endoscopes for minimally invasive management of lesions is the ability to determine the invasiveness of the lesion. The decision to perform an endoscopic resection of a lesion may be guided by the gross anatomy of the lesion (for example the shape and the size), or by using imaging modalities like ultrasound at the tip of the endoscope. However, with many lesions, a biopsy is taken and the lesion is subsequently managed, depending on the result of the biopsy, by open surgery or by additional endoscopy.
Colorectal cancer is the second most common visceral malignancy in the United States. More than 160,000 new cases are diagnosed each year; the disease causes approximately 60,000 deaths; and the lifetime average incidence for each individual is 6% (Bond, 1993). The two most effective ways to reduce the high mortality rate associated with this cancer are to diagnose it at an early stage, or to prevent it by detecting and resecting the precursor lesion of most of these cancers, the neoplastic adenoma (Atkin et al., 1993). Recent data, including results from the National Polyp Study (Winawer et al., 1991), support this claim and show that screening reduces colorectal cancer and saves lives. Polyps of the large bowel are very common and occur in more than 30% of people living in Western countries. More than 650,000 patients currently undergo colonoscopic polypectomy each year in the United States. Approximately 70% of polyps removed at the time of colonoscopy are true neoplastic growths or adenomas. When a small polyp is detected during screening proctosigmoidoscopy, a biopsy of the polyp should be performed to determine if it is an adenoma. The finding of a left colonic adenoma is an indication for colonoscopy to resect the polyp and search for additional synchronous lesions, since the incidence of synchronous adenomas is 40% to 50% (Winawer et al., 1997). Most patients with one or more polyps diagnosed by barium enema examinations should be offered colonoscopy to resect the detected polyps and to clear the colon of other lesions that might have been missed by the diagnostic X-Ray study (Hogan et al., 1977). Large sessile polyps almost always contain villous tissue with appreciable premalignant potential, and they tend to recur locally after colonoscopic resection. In many cases it is not clear whether these polyps can be safely or completely excised endoscopically, and the patients are referred for surgical resection. In some cases a polyp that appears benign at the time of colonoscopic polypectomy is later diagnosed by pathologic examination as a malignant polyp since it contains malignant cells that penetrate deep into the wall.
It is clear from the above that the use of local imaging can greatly improve the yield of endoscopy by better distinguishing between resectable and non-resectable lesions.
Endoscopic Ultrasound:
Endoscopic ultrasound (EUS) was introduced more than 20 years ago and is currently offered by major manufacturers of endoscopes (e.g. EVIS 130 ultrasonic videoscope, Olympus, Japan). Using EUS with mid-range frequencies (7.5 to 20 MHZ) enables the wall of the GI tract to be imaged as a five to nine layer structure of alternating bright (echogenic) and dark bands. These image layers largely correspond to the four histologic layers of the wall of the GI tract, but the redundant echo-related, non-histologic layers may cause mis-evaluation of the depth of tumor invasion (e.g. in the gastric wall, Yamada et al., 2001). Yet, the ease of using EUS, and its ability to distinguish between solid and nonsolid (e.g. cystic) structures, indicates its use for the following clinical indications:
1. Submucosal Abnormalities: EUS can be used to determine whether an abnormality is extrinsic (compression by normal organs or disease with all wall layers intact) or intramural. Basic distinctions can be made as to whether the tumor is cystic, vascular, or solid. It must be understood, however, that a precise histologic diagnosis, and the differentiation between benign and malignant tumors, are not possible with EUS imaging.
2. Cancer Staging: Diagnosis of cancer depends primarily on histologic or cytologic evaluation of biopsies. EUS cannot be reliably used to differentiate benign from malignant lesions, for example benign and malignant gastric ulcers. On the one hand, EUS was used to stage cancer of the esophagus, stomach, colon, and rectum (Tio, 1995). With the TNM staging classification, EUS images of the GI tract wall can be used to define the depth of tumor invasion (T), and in some cases involvement of regional lymph nodes (N). On the other hand, high-frequency EUS has a short depth of field and is not a good test for staging distant metastases (M). For staging the depth of tumor invasion of esophageal, gastric, and colorectal cancer, EUS has shown preoperative accuracy in the 80% to 90% compared with surgical pathology, while accuracy of staging regional lymph node metastases has been in the range of 70% to 80%.
This staging information can be used to determine wide operative resection, local endoscopic excision, non-operative management, adding preoperative adjuvant chemotherapy or radiation therapy (Lightdale, 1999; Hildebrandt and Feifel 1995). Lesions confined to the mucosa in patients who have increased operative risk may be treated endoscopically with various ablation modalities. Subepithelial lesions in the deep mucosa or small lesions in the submucosa can be removed during endoscopy if their location and size can be confirmed with EUS (Lightdale, 1996; Tada et al., 1996). Body imaging with CT or MRI of the chest and abdomen is usually the next step in staging the anatomic extent of GI cancers. The management of the cancer (i.e. by endoscopy, surgery, radiation, chemotherapy) is determined by the results of the endoscopy and body imaging, so the quality of local and body imaging plays a critical role in the management of these patients. MRI-enhanced endoscopy, providing high quality local as well as body images, can become the optimal tool for the initial management of these patients.
3. Endoscopic Needle Aspiration: EUS-guided fine-needle aspiration for cytology has been developed in an effort to improve the diagnosis of submucosal lesions, lymph nodes, and pancreatic masses (Wiersema et al., 1994). The initial experience indicates that this may be helpful in cancer staging, particularly in lymph node staging of non-small-cell lung cancer and in documenting pancreatic cancer. The overall complication rate from endoscopic fine-needle puncture is less than 1%. Experience is small, but the procedure seems at least comparable and in some cases more effective than CT-guided techniques for this purpose (Wiersema et al., 1994). MRI, with its superior soft-tissue contrast compared with ultrasound and CT, and its unique capability of scanning in any location and orientation, may become the modality of choice to guide fine-needle aspiration during endoscopy.
4. Pancreatic and Biliary Disease: EUS has been used to detect cysts, adenocarcinomas, and islet cell tumors of the pancreas. Lesions as small as 5 to 10 mm have been localized (Röösch et al., 1992). EUS-guided fine-needle aspiration has been used successfully to obtain material for cytology and tumor markers and to guide internal drainage of pancreatic pseudocysts. Staging of advanced pancreatic cancer can be achieved with accuracy in the 85% range. Ampullary tumors can also be staged, allowing endoscopic removal of ampullary adenomas in patients at high surgical risk. In severe pancreatitis, EUS is limited in its ability to differentiate between inflammation and neoplasm. Furthermore, early pancreatic structural changes cannot be detected by EUS in cases where other tests of pancreatic function and structure are normal.
Advantages of MRI:
Many of the advantages of MRI that make it a powerful clinical imaging tool are also valuable during interventional procedures. The most significant ones are the superior soft-tissue contrast, compared with all other imaging modalities, which allows for detection of early malignant growth; lack of ionizing radiation which enables its use for long, complex procedures; and its unique oblique and multiplanar imaging capabilities which enable the imaging of any plane or volume in the body, including non-planar surfaces. Its traditional drawbacks—high cost, slow imaging, and claustrophobic environment—are being eliminated through the introduction of relatively low-cost, fast, open scanners. These unique advantages make MRI a suitable imaging modality to enhance optical endoscopy.
Specifically, endoscopic MRI (EMRI) is not limited by some of the limitations of EUS mentioned above: The better soft-tissue contrast and the higher resolution may provide better correlation between imaging and histology. This is nicely demonstrated by a recent study that compared in-vitro MR scans using a small receiving coil and histologic sections in the diagnosis of specimens with suspected early gastric carcinoma (Yamada et al., 2001). While EUS is limited in its ability to differentiate between different pancreatic pathologies, for example chronic pancreatitis and pancreatic cancer, standard diagnostic MR scans are being proposed for the diagnosis of pancreatic cancer (Richter et al., 2001; Albert et al., 2000).
It should be noted that EUS is easier to perform as part of a standard endoscopy procedure, while EMRI requires the use of an MR-compatible endoscope and MR-scanner. However, unlike EUS, the combined use of high-resolution local scan with whole body scan is easily accomplished in standard MR scanners and may provide a “one shop” modality for comprehensive TNM (Tumor—Node—Metastase) cancer staging of different organs.
Intraluminal MRI
The use of intraluminal coils for MR studies dates back to 1984, when Kantor and colleagues used a catheter MR probe for in-vivo 31P MR measurements in the canine heart. Hurst and colleagues (1992) used simulation and experimentation to compare several configurations for intravascular MR receiver probes. They showed that optimal receiving sensitivity can be achieved by an opposed-coil configuration. The design is based on two coils, separated by a gap region, whose magnetic fields are in opposition to one another. The region between the two coils experiences a large magnetic flux and therefore achieves high sensitivity to regions between the coils and external to the diameter of the coils. Martin and Henkelman (1994) used similar configuration with an external diameter of 3.5 mm and a separation of 7.3 mm and achieved good receiving sensitivity up to a depth of 10-20 mm. This opposed-coil configuration has best performance when the coils lie parallel to the direction of the static magnetic field. However, it cannot be used when the coils are transverse to the direction of the static field, since the RF field has zero component in this direction. As most endoscopy procedures are done in organs with different spatial orientation inside the body, other types of RF probes with a full spherical coverage must be used.
Atalar and colleagues (1996) proposed the combination of two orthogonal coils (a quadrature coil design) to achieve better spatial coverage of the probe. They also proposed the use of printed circuit technology to manufacture low cost probes (U.S. Pat. No. 6,263,229). This arrangement eliminates the need to align the pickup coil along the main magnetic field of the magnetic resonance scanner. Kulling et al. (1997) tested similar configuration and demonstrated a good coverage around the endoscope. The quadrature coil design is useful to image slender lumen like blood vessels and provides adequate sensitivity in the radial direction around the probe, but poor longitudinal sensitivity and no “looking forward” capabilities.
Pioneering studies with EMRI have been published during recent years. Feldman et al. (1997) performed local staging of esophageal and rectal cancer (without pathologic correlation) using an MR-compatible endoscope with embedded receive-only MR coil. Kulling and colleagues (two studies published in 1998) found EMRI to be comparable to EUS for local staging of esophageal cancer in one study and of anal and distal colorectal tumors in the second study, but indicated more than 25% failure due to motion artifacts in the first study.
None of the published studies with intraluminal MRI used tracking since only optical tracking, which cannot be used with endoscopes, is commercially available for some open MR scanners. However, tracking is critical for optimal use of high resolution, local MR imaging, by enabling realtime prescription of the image plane. The EndoScout, described below, provides an optimal solution for MR tracking which is compatible for use with endoscopes.
The MRI Tracking System of U.S. Pat. No. 6,516,213
U.S. Pat. No. 6,516,213, cited above and incorporated in its entirety herein by reference, discloses a new tracking system which extends the existing limitations of MR tracking and provides an optimal tool for interventional MRI. Rather than using a tracking system based on external reference transmitters, the new tracking system uses electromagnetic fields that are generated by the MR scanner. A basic feature of MRI is the spatial encoding of the RF signal emitted from tissue protons by the spatially variable magnetic fields of the gradient coils of the scanner. The miniature sensors in the new tracking system measure the gradient magnetic fields; and the sensor's location and orientation are determined by comparing the measured fields to the known fields of the scanner.
The new MRI tracking system has significant advantages over existing ones. It can be easily used with any MR scanner as a “plug and play” option; there is no need for any mechanical integration (as needed with the Flashpoint optical system), electromagnetic adaptation, or change in the normal mode of operation of the scanner (as needed with General Electrics MR-Tracking technology, Dumoulin et al., 1993). Unlike other electromagnetic tracking methodologies that are used with other imaging modalities (e.g. the Carto system for X-ray catheterization, Biosense-Webster, Inc., Diamond Bar, Calif.; US-Guide and CT-Guide, UltraGuide Inc., Lakewood, Colo.) the new technique described in Pat. No. 6,516,213 is totally passive. It does not require any excitation of the sensors, nor the use of dedicated electromagnetic reference fields. Since the same spatial encoding mechanism is used by the scanner to reconstruct the image, and by the tracking system to determine the location of the sensor, there is no need to align different coordinate systems. In contrast to currently used optical tracking, the above new technique does not need an unobstructed line of sight to track the device (thus it can track non-rigid surgical tools and intra-body devices like catheters and endoscopes). It requires only one sensor to provide tracking of position and orientation, while competing tracking technologies (the Flashpoint, the MR-Tracking) determine only the location and thus require the use of at least two rigidly connected sensors to determine the device orientation.